Liquid crystal displays (LCDs) have characteristics such as small volume, light weight, low power consumption, low radiation, low manufacturing cost, etc., and thus have been widely applied in various electronic devices, for example, digital electronics such as displays, televisions, mobile phones, digital cameras, and the like. Among LCDs, thin film transistor liquid crystal display (TFT-LCD) is a main type of flat panel display (FPD). The drain current of a thin film transistor in saturation region is directly proportional to the width to length ratio of a channel, that is, the larger width to length ratio of a channel, the larger drain current of a thin film transistor in saturation region, which means that the wider the channel of a thin film transistor is, the better the performance of the device will be. However, due to the limitation of aperture ratio, the width of the channel of a thin film transistor is limited and thus cannot optimize the performance to the largest extent.